Routing and Wavelength Assignment (RWA) is a well-known problem for fixed grid optical networks while Routing and Spectrum Assignment (RSA) is its equivalent term to the same problem for flexible grid optical networks. In fixed grid optical networks, wavelengths are spaced apart from each other according to a wavelength spectrum grid defined by International Telecommunication Union (ITU) in ITU-T G.694.1 (February 2012), “Spectral grids for WDM applications: DWDM frequency grid,” the contents of which are incorporated by reference. In flexible grid optical networks, which is also described in ITU Recommendation G.694.1 “Spectral grids for WDM applications: DWDM frequency grid” (February 2012), each signal can be allocated to spectrum with different widths optimized for the bandwidth requirements of the particular bit rate and modulation scheme of the individual channels. The ultimate objective of RWA or RSA is to find a wavelength or spectrum assignment on a route for a particular channel in the optical network, such assignment and routing being optimal in some manner.
Most conventional approaches to RWA deal with routing and wavelength assignment as two sub-problems. The majority of routing solutions is constrained-shortest-path while wavelength assignment is done by simple random-fit or first-fit techniques. There are various adequate solutions to RWA in the fixed grid domain; however, there is none or few complete solutions for RSA in the flexible grid domain. At present, the research for RSA in the flexible grid domain is still focused on the problem description or special case solution without describing a complete algorithm or technique to solve ultimately RSA. Without adequate RSA techniques, the transition from the fixed grid to the flexible grid is expected to be slow and difficult despite the benefits of the flexible grid.
Recalling that the conventional approaches solve routing and wavelength assignment as two problems, namely routing first and then wavelength assignment secondly. Disadvantageously, the resulting solution may not be globally optimal, since another route may need to be chosen as a result of the wavelength assignment analysis. Traditional searching techniques for a paired solution of both the routing and wavelength assignment sub-problems are very exhaustive, time-consuming, and suffer from computational complexity. Using optimization search engines is an expensive solution, adds another layer of complexity, and its solution may not be feasible until relaxing some constraints of the target functions.